The invention relates to well logging for evaluating the resistivity of earth formations through which a borehole passes, and it relates particularly to methods and apparatuses for electrical logging using azimuthal electrodes.
Logging apparatuses are known that detect the resistivity of formations in a plurality of azimuthal directions around the axis of the borehole. For example, British patent GB 928 583 (British Petroleum Company Limited), describes an array of azimuthal electrodes that are distributed circumferentially around the periphery of a sonde. Measurement currents are emitted by the azimuthal electrodes. A guard current is emitted by a guard electrode which surrounds the azimuthal electrodes in order to focus the measurement currents.
Another logging apparatus having azimuthal electrodes is described in French patent application number 91 14701 filed Nov. 28, 1991. The sonde comprises an array of azimuthal current electrodes together with guard electrodes disposed on either side of the array of azimuthal electrodes. In addition, the sonde comprises monitor electrodes associated with the current electrodes and the guard electrodes. Servo-control loops or circuits sensitive to the potentials detected on the monitor electrodes control the measurement currents.
Under certain circumstances these techniques can give rise to problems, particularly if the emission of one of the azimuthal currents is abnormal due to faulty operation of the apparatus, which then has the effect of disturbing all of the measurements. In other cases, if the borehole fluids have high resistivity, then it is difficult to obtain accurate resistivity measurements.
An object of the invention is to provide a technique of measuring azimuthal resistivity that is relatively insensitive to instrument limitations.
Another object of the invention is to provide a logging technique using azimuthal electrodes making it possible to obtain satisfactory measurements under conditions of high-resistivity borehole fluids.
Still another object of the invention is to provide a focused electrical logging technique having two operating modes that can be selected according to circumstances.